jsonrpc: when None is set as default value Bridge.py fails

When None is set as default value for parameter which uses custom type
Bridge.py fails.

Bug-Url: https://bugzilla.redhat.com/1143854
Change-Id: Ia1a1cea87e62952f9675cba3b9d8e891985c8333
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/33033
Reviewed-by: Oved Ourfali <oourfali@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>